ext: Silabs Gecko use a different config for uart

Use CONFIG_UART_GECKO instead of CONFIG_SERIAL_HAS_DRIVER to compile
the low level uart driver.

Signed-off-by: Diego Sueiro <diego.sueiro@gmail.com>

ext/hal/silabs/gecko/CMakeLists.txt

@@ -25,7 +25,7 @@ zephyr_compile_definitions(
 zephyr_sources(                                  emlib/src/em_system.c)
 zephyr_sources_ifdef(CONFIG_HAS_CMU              emlib/src/em_cmu.c)
 zephyr_sources_ifdef(CONFIG_GPIO_GECKO           emlib/src/em_gpio.c)
-zephyr_sources_ifdef(CONFIG_SERIAL_HAS_DRIVER    emlib/src/em_usart.c)
+zephyr_sources_ifdef(CONFIG_UART_GECKO           emlib/src/em_usart.c)
 zephyr_sources_ifdef(CONFIG_SOC_SERIES_EFM32WG   Device/SiliconLabs/EFM32WG/Source/system_efm32wg.c)
 zephyr_sources_ifdef(CONFIG_SOC_SERIES_EFR32FG1P Device/SiliconLabs/EFR32FG1P/Source/system_efr32fg1p.c)
 zephyr_sources_ifdef(CONFIG_SOC_SERIES_EFM32HG   Device/SiliconLabs/EFM32HG/Source/system_efm32hg.c)